This invention relates to a process for determining the current state of a lubricant that lubricates at least one pair of frictional coupling elements which cooperate at least some of the time to transmit a torque, and, in particular, for determining the current state of the transmission oil for lubrication of two wet-running frictional coupling elements of a transmission, wherein the cooperating coupling elements have certain slip characteristics at a specific quality state of the lubricant.
In wet-running clutches comprised of two frictional coupling elements selectively engagable to transmit a torque, xe2x80x9cmicroslipxe2x80x9d is present between the coupling elements even when engaged in the lower portion of the specified load range. This microslip increases as the load increases, i.e. as the torque to be transmitted increases, rising slowly but steadily into the region of xe2x80x9cmacroslipxe2x80x9d. When the frictional coupling element lubricant degrades due to the factors discussed below, the slip characteristics of the frictional coupling elements change and microslip no longer occurs at low torque, leading to an abrupt transition to macroslip when a certain torque threshold is exceeded.
Lubricants are subject to degradation caused by temperature fluctuations, length of time the lubricant is used, type and manner of use and contamination of the lubricant. Degraded lubricants are ineffective at protecting the elements they lubricate from damage caused by friction. Additionally, degraded lubricant can lead to damage to components connected to the elements the lubricant protects. For example, components within a transmission, such as gears, synchronizers and sliding sleeves can be damaged when the frictional coupling elements in the clutch connected to the transmission are lubricated by degraded lubricant. Consequently, lubricants must be replaced after an appropriate period of use. With regard to engine lubricants in motor vehicles, it is customary to replace such lubricant after every 15,000 km or 30,000 km driven or after a particular amount of time, e.g. every 12 months. To alert the driver of a motor vehicle that it is time for an oil change, it is known to provide appropriate visual displays in the passenger compartment which are activated in response to signals (typically generated by an on-board computer) based on the mileage driven. However, such methods do not account for premature degradation in the lubricants caused by temperature variations, differences in individual driving behavior, or contamination to the lubricant caused by seal leaks or other factors. Such premature degradation could occur prior to the end of the inspection interval, causing damage as previously discussed.
DE 19,644,029 A1 discloses a process for determining the functional suitability of a lubricant that lubricates slip-controlled clutches using a test vessel. The process tests different lubricants for their suitability for use in slip-controlled clutches. The process does not, however, determine the current state of a lubricant that is already being used to lubricate two cooperating frictional coupling elements.
Accordingly, there exists a need for a method of determining the current state of lubricant which is lubricating at least one pair of frictional coupling elements that transmit a torque when engaged.
It is an object of the invention to provide a method for determining the current state of a lubricant that lubricates at least one pair of frictional coupling elements arranged to cooperate and transmit a torque when engaged.
It is another object of the invention to prevent damage to frictional coupling elements and/or components connected to the frictional coupling elements resulting from inadequate lubrication of the frictional coupling elements.
The invention provides a method for determining the current state of a lubricant that lubricates at least one pair of frictional coupling elements which transmit a torque when engaged. The current slip characteristics of the coupling elements are determined and compared to reference slip characteristics which correspond to specific quality states of the lubricant. The current state of the lubricant is thereby determined.
Preferably, the reference slip characteristics are known for selected operating conditions. These selected operating conditions may include torque transmission, temperature and force exerted between the frictional coupling elements.
In a preferred embodiment, if an improper quality state of the lubricant is detected, a vehicle operator is signaled.
In another preferred embodiment of the present invention, the current slip characteristics are determined from the results of measuring the rotational speed of each coupling element.
In a further preferred embodiment, the comparing of current slip characteristics to the reference slip characteristics is performed by a processor such as a digital processor. In a highly preferred embodiment, the reference slip characteristics are stored in a memory associated with the processor.
In a highly preferred embodiment, determining the current state of the lubricant includes generating a signal when the value of the current slip characteristics differs from a reference slip characteristic value by a predetermined amount.
In this manner, a method of determining the current state of a lubricant lubricating two frictional coupling elements is provided and an operator may be signaled when the lubricant becomes degraded so the lubricant may be replaced before damage occurs.
Additional features and advantages of the invention appear in the following description of preferred embodiments of the invention, reference being made to the accompanying drawing which is incorporated and constitutes a part of this disclosure.